1. Field of the Invention
The present invention generally relates to the design and construction of electrical inductors and, more particularly, to a method for designing inductor structures which are compatible with silicon technology.
2. Description of the Prior Art
Miniaturization of electronic circuits is a goal in virtually every field, not only to achieve compactness in mechanical packaging, but also to decrease the cost of manufacture of the circuits. Many digital and analog circuits, including complex microprocessors and operational amplifiers, have been successfully implemented in silicon based integrated circuits (ICs). These circuits typically include active devices such as bipolar transistors and field effect transistors (FETs), diodes of various types, and passive devices such as resistors and capacitors.
One area that remains a challenge to miniaturize are radio frequency (RF) circuits, such as those used in cellular telephones, wireless modems, and other types of communication equipment. The problem is the difficulty in producing a good inductor in silicon technologies that is suitable for RF applications. Attempts to integrate inductors into silicon technologies have yielded either inductor Q values less than five or required special metalization layers such as gold. This is in part due to the fact that the inductance of spiral on-chip inductors is very difficult to predict, so that design changes to increase inductance and decrease resistance can only be verified using hardware redesigns.
The objective of high-Q inductance designs is to increase inductance and decrease resistance, while keeping parasitic capacitance to a minimum so that high oscillation frequencies can be achieved. Several techniques for doing this can be used. One way is to use wide metal line-widths; however, this increases the inductor area and the parasitic capacitance associated with the structure. Therefore, the self-resonance frequency of the inductor decreases, thereby limiting its useful frequency range. Since the Q is directly proportional to frequency and inversely proportional to the series loss of the inductor, the metal line widths cannot be chosen arbitrarily large.